Molecular Fluorescence and Phosphorescence (5)

Question 1

What is photoluminescence?

Answer 1

process where molecules are elevated to excited state; excess energy emitted as radiation of wavelength different than wavelength of absorption; excitation/de-excitation process involving photons

Question 2

de-excitation process that occurs after excitation by photons

Answer 2

photoluminescence

Question 3

de-excitation process that occurs after excitation by high energy particles

Answer 3

radioluminescence

Question 4

de-excitation process that occurs after excitation by chemical process

Answer 4

chemiluminescence

Question 5

what is the total spin quantum number S?

Answer 5

sum of net spin

Question 6

What is S for uncharged molecules?

Answer 6

0

Question 7

How is multiplicity calculated?

Answer 7

2S + 1 = x where x is like singlet, triplet, etc

Question 8

What is a singlet state?

Answer 8

multiplicity of one

Question 9

What is S0?

Answer 9

ground

Question 10

What is S1?

Answer 10

excited state 1

Question 11

What is S2?

Answer 11

excited state 2

Question 12

What is a triples tate?

Answer 12

formed when an excited electron reverses its spin

Question 13

what is the quantum mechanical selection rule for single and triplet states?

Answer 13

ΔS=0

Question 14

What is the time scale for electronic absorption?

Answer 14

10^-15 seconds

Question 15

What is a Jablonski diagram?

Answer 15

qualitative depiction of absorption and emission processes

Question 16

What is vibrational relaxation?

Answer 16

molecule rapidly and nonradiactively dissipates excess vibrational energy as heat by collision with solvent molecules

Question 17

What is the time scale for vibrational relaxation?

Answer 17

10^-12 seconds

Question 18

What is internal conversion?

Answer 18

molecule intermolecularly passes from original excited state to a lower one of the same multiplicity

Question 19

What is the time scale for internal conversion?

Answer 19

10^-12 seconds

Question 20

What is fluorescence

Answer 20

process of photon emission for de-excitation S1 to S0

Question 21

How does the wavelength of fluorescence emission compare with wavelength of initial absorption and why?

Answer 21

at lower energy and longer wavelength because of vibrational relaxation

Question 22

What is the 0-0 transition?

Answer 22

fluorescence and absorption between two electron states that have at least one transition of the same energies; between v” = 0 S0 and v’ = 0 S1

Question 23

What is the time scale for fluorescence?

Answer 23

10^-9 seconds

Question 24

What is external conversion?

Answer 24

energy transfer between excited molecules and solvents

Question 25

How does external conversion differ from internal conversion?

Answer 25

external is de-excitation between excited molecule and other solvents, internal is lowering from original excited to a lower excited state of the same multiplicity (S2 to S1)

Question 26

What is intersystem crossing?

Answer 26

molecule in excited state may change spin multiplicity from single to triplet

Question 27

What is the time scale for intersystem crossing?

Answer 27

varies but can be 10^-10 seconds with heavy or paramagnetic molecules

Question 28

What is phosphorescence?

Answer 28

From triplet, molecule rapidly relaxes to lowest vibrational level

Question 29

From which spin state does phosphorescence occur?

Answer 29

T1 to S0

Question 30

What is the time scale of phosphorescence?

Answer 30

10^-3 to 10 seconds

Question 31

Why is phosphorescence frequently carried out at liquid nitrogen temperatures?

Answer 31

cannot be seen in fluid solutions at room temperature because there are many deactivation processes that have faster rate constants

Question 32

What is quenching?

Answer 32

non radiative process that depopulates S1 or T1 levels and competes with fluorescence and phosphorescence

Question 33

What governs the observed intensities in electronic processes?

Answer 33

overlap of vibrational wave functions in upper and lower states according to frank-condos principle (about geometry changes)

Question 34

What is the stokes loss/shift?

Answer 34

difference in energy between position of maximum emission and 0-0 transition

Question 35

what does stokes shift measure?

Answer 35

change in geometry of equilibrium configurations of ground and excited states

Question 36

What kinetic rate law describes fluorescence?

Answer 36

pseudo first order kinetics

Question 37

What is the fluorescent quantum yield?

Answer 37

fraction of total number of photons absorbed that result in fluorescent emission

Question 38

How is fluorescent quantum yield defined?

Answer 38

total number of fluorescent photons emitted / total number photons absorbed

Question 39

How is the steady state fluorescent lifetime related to the kinetic rate constants of the de-excitation process?

Answer 39

inverse

Question 40

How is the fluorescent quantum yield related to the steady state lifetime?

Answer 40

quantum yield = steady state lifetime divided by radiative lifetime